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A Materials Science Based Model for the Life of Lead Free Solder Joints in Thermal Cycling
Keywords: reliability , model, solder
The present work is part of a comprehensive effort to develop a model for the performance of SnAgCu solder joints in thermal cycling based on well established materials science principles. The properties of practical joints are unstable after reflow and continue to change during long term aging and thermal excursions. This complicates modeling considerably, and a major effort is currently focused on the development of the associated constitutive relations. Thermal cycling tends to lead to recrystallization of the Sn grains across the high strain region before failure. We have shown crack growth to initiate almost immediately during thermal cycling, albeit at a rate low enough that this may be mistaken for an extended crack initiation time. Crack growth was seen to accelerate strongly after 25-50% of the total life, coinciding with the formation of a recrystallized band across the joint, and cracks then propagated rapidly along the new grain boundaries. Life thus depends directly on the rate of recrystallization. Results on the factors governing this, including coarsening of secondary precipitates and the rate of inelastic strain energy deposition as well as competing recovery mechanisms, will be presented and quantitative modeling of these will be discussed.
Luke Wentlent, Graduate Research Associate
Binghamton University
Binghamton, NY

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